The present invention relates to a fluid micro-metering device, and more particularly to an improved configuration for an electrostatic mechanically actuated fluid micro-metering device having an array of fluid chambers with orifices for metering fluid, that achieves a higher pitch density for the chamber array.
Micro-metering of a fluid is useful in many applications and is especially important where fluid dosage is critical, for either functional or economic reasons. For example, an ingredient may be precisely metered in a production line to achieve a desired product quality, or an exotic material may be metered accurately to reduce cost.
One such application involves the micro-metering of ink from an impulse or drop-on-demand (DOD) ink jet printing device. Ink jet printing technology has revolutionized the office and home printer markets over the last two decades and is increasingly being used in industrial printing applications. Impulse Ink jet printing is performed by ejecting ink droplets from orifices or nozzles in the print head, such that the droplets travel to and are deposited on a substrate, forming a printed image. The print head associated with an ink jet printer typically comprises chambers aligned in an array, each chamber having at least one orifice for ejecting ink. Actuation devices associated with the chambers are energized and de-energized to create pressure changes in the chambers, resulting in the ejection of droplets of ink from the orifices.
For apparatus involving an array of fluid chambers, pitch is defined as the density of dots (or droplets of fluid) that are ejected from the array, expressed as drops per inch (DPI). The pitch of the array, e.g., print head, is directly related to how closely aligned the ink chambers of the linear array are. Thus, a print head having a high pitch translates into better printing resolution and clarity (greater DPI). High printing resolution is demanded by such applications as bar code printing, carton and letter labeling, business form printing, and higher resolution printing on substrates such as garments, packages and various parts.
Image formation can be controlled in impulse ink jet printers by selectively energizing and de-energizing actuators that change the pressure in the ink chamber, resulting in the ejection of ink through the orifices. One type of electromechanical actuator that has been used in ink jet printing is a piezoelectric transducer, for example, based on lead-zirconate-titanate. One class of piezoelectric print head design adheres the piezoelectric element to a wall of the chamber, so that the application of voltage to the piezoelectric causes distortion and deformation of the wall, thereby creating a pressure pulse in the chamber to eject the ink droplet. Another class involves utilizing the piezoelectric element itself as the chamber wall.
Piezoelectric elements, however, are brittle, and piezoelectric actuators often require precise machining to manufacture the actuators at the required dimensions. Another disadvantage is that many piezoelectric actuators need to be attached to a membrane with an adhesive or similar agent. Such machining and bonding processes require significant time and labor, and are subject to poor manufacturing tolerances. There is often an inherent limitation associated with machining capability, accuracy and tolerances concerning the manufacturing and construction of high pitched piezoelectric print heads. Further, piezoelectric actuators pose limitations in applications requiring higher resolution ink jet printing because piezoelectric transducers are prone to material defects and distortions introduced by manufacturing variability, which in turn leads to electromechanical inefficiencies. Consequently, the piezoelectric electromechanical impulse ink jet technology is limited in its ability to meet the demands of high0resolution imagining applications.
An example of such a piezoelectric actuated print head is disclosed in U.S. Pat. No. 5,227,813 (Pies et al.) showing a piezoelectric side wall actuated print head having a conductive surface adhered to and separating a first side wall section of an inactive material from a second side wall piezoelectric section, wherein the second side wall undergoes a shear-like motion to pull the first side wall section, thereby pressurizing the ink chamber.
In order to overcome some of the disadvantages associated with piezoelectric actuators, electrostatic mechanical actuators have also been used in impulse ink jet print heads. Such electrostatic actuators can comprise thin plates (also called diaphragms or membranes) formed adjacent to the ink chambers. In such an arrangement, a chamber wall that contains the ink can comprise a plate, which forms the actuator. When a time varying electric field is applied to an electrode in close proximity to the plate, the wall is deflected by the electrostatic force exerted between the plate and the electrode, producing a pressure disturbance in the chamber, thereby ejecting a drop of fluid from the chamber through an orifice.
For example, U.S. Pat. No. 4,520,375 (Kroll) discloses a fluid injector having a pair of capacitor plates spaced by an insulator, wherein a varying electric field between the plates sets a silicon membrane into mechanical motion causing fluid to eject through a nozzle.
U.S. Pat. No. 5,534,900 (Ohno et al.) discloses an electrostatically actuated ink jet print head having multiple layers and a plurality of nozzle openings communicating with independent injection chambers, wherein a membrane is positioned on a bottom wall of the injection chamber. In such a configuration, the driving voltage to actuate the membrane increases approximately exponentially as the pitch of the ink jet head is increased.
A disadvantage of prior art designs involving electrostatically actuated fluid jetting devices is that the membrane is orientated so that the pitch of the array is dependent upon the areal dimensions of the membrane (i.e., membrane length and widthxe2x80x94not thickness). In other words, the membrane comprises the top or bottom chamber wall, or even the back wall opposed to the orifice plate. Such an orientation limits pitch, a critical dimension of the chamber array, in that the pitch decreases as the membrane width increases, deteriorating the resolution of the device. The applied or driving voltage required to actuate the membrane also increases approximately exponentially as the pitch of the fluid device is increased.
What is desired therefore is a configuration for an electrostatic mechanically activated micro-metering device that overcomes the above disadvantages.
Accordingly, it is an object of the present invention to provide an electrostatic mechanically actuated fluid micro-metering device, such as an impulse ink jet print head, that achieves a higher density pitch, without requiring a substantially exponential increase in the applied voltage.
Another object of the present invention is to provide an electrostatic mechanically actuated fluid micro-metering device, such as an impulse ink jet print head, including an array of chambers, wherein the width of each chamber is substantially independent from the areal dimensions of the electrostatic membrane provided within that chamber.
Another object of the present invention is to provide an electrostatic mechanically actuated fluid micro-metering device, such as an impulse ink jet print head, including an array of chambers, wherein the pitch of the array is substantially independent from the areal dimensions of the electrostatic membrane provided within each chamber, and wherein each chamber has a width as low as about 50 micron to achieve about a 300 DPI resolution, or preferably as low as about 25 microns to achieve about a 600 DPI resolution.
The present invention is an electrostatic mechanically actuated fluid micro-metering device, such as an impulse ink jet print head, having an electrostatically activated membrane that is oriented on a side wall of a fluid chamber and between adjacent chambers within a chamber array. This design eliminates the prior art inter-relationship and dependence between the areal dimension of the membrane and the pitch of the chamber array, so that higher resolution at moderate operating voltages may be achieved.
The present invention comprises: an electrostatic mechanically actuated fluid micro-metering device comprising an array of fluid chambers having a width (transverse axis); the array having a pitch substantially determined by the chamber width; wherein the chambers have one or more thin walls (or membranes) able to deform in the direction of a deformation axis, under the influence of an electrostatic force created by an electrical potential difference between such thin wall and an adjacent and closely spaced fixed electrode; the membrane deformation axes are substantially parallel to the transverse axes of the chambers.
The invention and its particular features will become more apparent from the following detailed description with reference to the accompanying drawings.